Method for manufacturing a steel sheet and product obtained thereby

ABSTRACT

A killed steel having a particular composition is heated, subjected to hot rolling such that the finishing temperature is 750*C or less, and subjected to recrystallization treatment. A steel sheet thus obtained has excellent press formability.

United States Patent Gondo et al.

[451 July 29, 1975 METHOD FOR MANUFACTURING A STEEL SHEET AND PRODUCTOBTAINED TI-IEREBY Inventors: Hisashi Gondo; I'Iiroshi Takechi;

Tsuyoshi Kawano, all of Kisarazu; I-Iiroaki Masui, Kimitsu, all of JapanAssignee: Nippon Steel Corporation, Tokyo,

Japan Filed: Dec. 11, 1973 Appl. No.: 423,821

Foreign Application Priority Data Dec. 23, 1972 Japan 47-128853 Dec. 23,1972 Japan 47-128854 US. Cl 148/12 C Int. Cl. C21D 9/48 Field of Search148/12 C Primary ExaminerW. Stallard Attorney, Agent, or Firm-WatsonLeavenworth Kelton & Taggart [57] ABSTRACT A killed steel having aparticular composition is heated, subjected to hot rolling such that thefinishing temperature is 750C or less, and subjected torecrystallization treatment. A steel sheet thus obtained has excellentpress formability.

14 Claims, No Drawings BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to methods for manufacturing steelsheet, and more particularly. to methods for manufacturing steel sheetdemonstrating excellent deep drawing properties.

2. Description of the Prior Art One of the most important properties ofa steel sheet for use in complex press formability is a capability fordeep drawing or deep-drawability. This capability becomes better as thenumber of crystallographic planes {111} parallel to the surface of thesteel sheet is increased or the number of planes {100} is decreased, andcan be estimated by means of Rankford value or Conical cup value.

In an attempt to obtain a steel sheet having deep-drawability, there hasbeen proposed a method for cold-rolling a hot-rolled steel sheet under asuitable reduction rate and thereafter annealing the same above therecrystallization temperature.

It is a common practice at present to effect a hotrolling above A pointor usually above 860C for a steel sheet to be used for deep drawing'lnthis case, however, the deep-drawability is extremely inferior to thatof a cold-rolled steel sheet since the axial density of the plane {11 1}parallel to the surface of the steel sheet is as low as about 1 and itis arranged under substantially random conditions from thecrystallographic viewpoint. In a special case, a hot-rolled steel sheethaving a low yield point may be manufactured by lowering the finishingtemperature down to about 800C. In this case, the steel sheet becomessoft but the axial density of the plane {100} parallel to its surface isremarkably increased and thus the deep-drawability of the sheet becomesso low that it can not be used as a steel sheet for deep drawing.Accordingly, as far as deep drawing is concerned, cold-rolled steelsheet which can be manufactured by means of the first-mentioned methodis suitable. The disadvantage of cold-rolled steel lies, however, in itshigh production cost due to complicated manufacturing steps as comparedwith the manufacturing steps used for making hot-rolled steel sheet.

Recently, there has been conducted a study for improving thedeep-drawability by effecting, after rolling at warm temperatures, atreatment for recrystallization without cold rolling. When it is desiredto obtain excellent deep-drawability by means of rolling a lowcarbonsteel at warm temperatures, it is inevitable, according to the result ofthe inventors study, to bring the finish temperature down to 550C orless, otherwise the object can not be accomplished.

Rolling at such low temperatures will not only greatly reduce commercialproductivity extremely but will also result in increase in rolling load.In view of this, it is desirable that evenwhen rolling at warmtemperatures is effected, the rolling temperature should be capable ofbeing increased to the highest level possible yet providing a steelsheet having excellent deep-drawability.

SUMMARY OF THE INVENTION As the result of various studies made by theinventors, they have succeeded in manufacturing a steel sheet havingexcellent deep-drawability by limiting the composition of the steel andthe conditions for its hot rolling to a material of a particular rangeand effecting the hot rolling followed by recrystallization treatmentwithout a step of cold rolling.

It is therefore an object of the invention to provide a method formanufacturing a steel sheet having excellent press formability by meansof hot-rolling at low temperatures followed by recrystallization withouta step of cold-rolling after the hot rolling.

According to the invention, there is provided a method for manufacturinga steel sheet having excel.- lent press formability which comprisesheating a killed steel composed of not more than 0.10% C, 0.05% to 1.0%Mn, 0.005% to 0.15% A1, at least one member of the group consisting ofTi, Nb and Zr, and the rest Fe and unavoidable impurities, the Ti%/4Nb%/7.8 Zr%/7.6 being more than the C% in the steel, subjecting the sameto hot rolling such that the finishing temperature is 750C or less, andthereafter effecting a treatment for recrystallization. Also provided isa method according to the above method in which said killed steelfurther contains at least one member selected from the group consistingof not more than 0.15% P, not more than 0.10% W, not more than 0.10% Mo,not more than 0.3% Cr,.not more than 0.3% Cu, and not more than 0.4% Si.

Still further is provided a steel sheet having excellent pressformability produced by the above mentioned method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS sheet having a thickness of 2.7mm was manufactured under the conditions shown in Table 2. The conicalcup value tested thereon is also shownin Table 2.

Table 1 Chemical composition of steel materials C Si Mn P s N 0 Al T1 47'8 Table 2 Conical 'cup value (mm) ol'stccl sheets Condition ofmanufacture Conical cup (mm) Finishing Heat treatment Temperature afterrolling A B Conventional 900C None (650C.coiling) 85.4 mm 86.5 mm method820C None (700C coiling) 87.5 88.5 750C 85.0 89.0

700% 80.3 88.1 This Box annealing This invention 600C (700C X 4 hr) 76.4imention 871 Remarks:

Thickn s", 2.7 mm

('onicul.cup blank diameter; 108 mm .As seen from Table 2,deep-drawability decreases as soon as the finishing temperature, whichis above 900C decrease from 900C which is the usual finishing but deepdrawability increases as the finishing temperature is further decreased.It is then obvious that the B steel, or control, has deep-drawabilitywhich becomes equal or superior to that of steel finished at 900C whenthe finishing temperature is reduced to about 550C or less. Contrary tothis, the A steel of the invention, when subjected to hot rolling suchthat the finishing temperature is 750C or less, and then to annealing,shows deep-drawability which is superior to that of steel finished at900C and which is extremely superior to that of the B steel at the samefinishing temperature.

The subsequent study revealed that in order to manu facture a steelsheet having such excellent deep-drawability, it is necessary to satisfythe conditions shown below.

One of such conditions is that the chemical composition of the steelmaterial requires such combination of elements as to enhance therecrystallization temperature indicated for that of the cold rolledmaterial. At the same time it is necessary not only to enhance therecrystallization temperature but also to form a deposit or precipitateat hot or warm temperature rolling which is suitable for developing theaxial density of the plane {1 l 1} parallel to a surface of the steelsheet during the recrystallization treatment.

For the purpose of accomplishing the above, a composition of the steelaccording to the instant invention should be that C is not more than0.10%; Mn is 0.05 to 1.0%; A1 is 0.005 to 0.15%, and that when Ti, Nb orZr is to be added singly, Ti%/C% 4, Nb%/C% 7.8 or Zr%/C% 7.6. is to besatisfied respectively. If the Ti, Nb or Zr are added in combination,the characteristic properties of the product becomes more stabilized.When they are added in such combination, Ti%/4 l- Nb%/7.8 l'- Zr%/7.6 C%in the steel must be satisfied. If necessary, not more than 0.15% P, notmore than 0.10% W, not more than 0.10% Mo, not more than 03% Cr, notmore than 0.3% Cu, and not more than 0.4% Si should be added singly orin combination. As for the content of C, it should be up to 0.10%. If itis more than 0.10%, it will not only deteriorate the deep-drawability ofsteel sheet, but will also increase the amount of Ti, Nb or Zr to beadded, which gives rise to an increase in the cost. Moreover, theincrease of C content will result in elevation of the heatingtemperature necessary for solubilizing TiC, NbC' or ZrC which extremelylowers the productivity of hot rolling. For these reasons the content ofC should preferably be 0.03% or less, if the strength of the product isnot stressed very much. Deep drawability is further enhanced bysatisfying C 0.02% by means of vacuum degassing treatment and the like.

Mn is necessary in an amount of at least 0.05%, and preferably at least0.1% for the purpose of deoxidation and prevention of hot brittleness.However, deep-drawability is lowered when the amount of Mn is more than1 .0%. If the steel sheet is not stressed, it should preferably be 0.5%or less in view of the deep-drawability.

Al is necessary in an amount of at least 0.005%, and preferably at least0.01%, for deoxidation. If it is, however, more than 0.15%, itdeteriorates the cleanliness of the steel and degrades the surfacecondition of the steel.

P serves to enhance the anti-weather property as well asdeep-drawability but it should be added in an amount of 0.15% or less,since an amount more than 0.15% will harden the steel.

W and Mo will promote deep-drawability. The amount of W and Mo to beadded should be not more than 0.10% since an amount more than 0.10% willadd substantially nothingto the effect thereof.

Cr and Cu will promote the anti-weather property. The amount of Cr andCu to be added should be not more than 0.3% since an amount more than0.3% will add substantially nothing to the effect thereof.

Si to be added should be not more than 0.4%. If it is more than 0.4%. itwill impart an adverse effect upon the ductile property of the material.

In addition to the above limitation for each component of the steel,another condition to consider in carrying out the method of the instantthe invention is the control of the rolling conditions. In order toeffectively generate finely-divided deposits or precipitates which areconsidered to be necessary for enhancing the recrystallizationtemperature and for forming an aggregative structure desirable fordeepadrawability in the recrystallization treatment after hot rolling,that is, in order to allow the TiC in case of Ti-added steel, or the NbCin case of Nb-added steel, etc. to effectively deposit or precipitate,or in order to allow the cold strain to remain in the rolled material,it is necessary to keep the finishing temperature at 750C or less. If itis more than 750C, the deep-drawability of the material subjected to hotrolling, followed by recrystallization treatment. is inferior to that ofthe hot rolled material subjected to finish rolling at a temperaturemore than A;, point which has been a common practice. The finishingtemperature will promote the deep-drawability as it decreases as shownin Table 2. It is thus preferable to keep the finishing temperature aslow as possible. However, a finishing temperature below 400C is notpracticable since it will increase the deformation resistance of themill to such an extent that the rolling power must be made greater, andin addition it will fail to provide a better deep drawing. In additionto the control or limitation of the finishing temperature, when thetotal reduction rate below 800C is made greater, preferably made 40% ormore of the thickness of the material at 800C, the deep-drawability isfurther promoted. In conventional continuous finish rolling, thelimitation of the finishing temperature to 750C or less will give asteel sheet having better deep-drawability than the conventional case inwhich the rolling is finished at more than A point.

In this invention, after the above rolling is over, therecrystallization treatment is effected so as to release the strainaccumulated in the rolled material as well as develop the axial densityof the planes {1 1 l} which are parallel to the surface of the steel andwhich are desirable for the deep-drawability. This recrystallizationtreatment is conducted by heating at temperatures ranging from therecrystallization temperature up to A point, or by such suitable methodsas box annealing, open coil annealing, continuous annealing, etc. Whenthe heating is effected below the recrystallization temperature, notonly is the strain can not fully be released but also that anaggregative structure desirable for deep drawing can not be developed.Moreover, when the annealing is effected above A point, the structurebecomes substantially random from the crystallographic 6 viewpoint,which results in lowering of deep-drawability.

EXAMPLE A steel material having a chemical composition shown in Table .3was melted in a converter, from which a slab was manufactured by theconventional steps. The slab was heated at about 1,200C, then subjectedto hot rolling in a continuous hot rolling mill at three levels offinishing temperatures, 550C, 650C and 740C, followed by water coolingand then, coiling. Thereafter the steel was subjected to a box annealingat 700C for 4 hours or to a continuous annealing at 850C for 5 minutes.The conical cup value and the axial density of-the planes {1 11} and{100} parallel to the surface with respect to'the above-treated steelsheet and the same steel sheet differing only by having subjected'to hotrolling at the conventional finishing temperature, 900C, are shown inTable 4 and Table 5, respectively. The thickness of the sheet was 2.7 mmin both cases.

As is obvious from Table 4, the steel sheet of the invention has thehigh axial density of the planes {1 l l} and excellent deep-drawabilityas compared with that of the prior art, said deep-drawability beingcomparable to that of cold rolled steel sheet.

Table 3 Chemical composition in Example C Si Mn P S p O A1 Ti Control C0.062 0.01 0.26 0.01 1 0.013 0.0024 0.030 0005 steel Steel D 0.008 0.020.27 0.009 0.009 0.0051 0.005 0.026 0.1 l of this E 0.007 0.03 0.200.019 0.010 0.0062 0.006 0.018 invention F 0.008 0.02 0.32 0.012 0.0070.0052 0.006 0.024

O 0.015 0.03 0.19 0.015 0.013 0.0049 0.005 0.015 0.06 H 0.008 0.02 0.820.017 0.012 0.0043 0.005 0.074 0.18 1 0.010 0.02 0.17 0.013 0.012 0.00410.005 0.032 0.12 J 0.007 0.02 0.21 0.074 0.008 0.0037 0.006 0.029 0.07 K0.01 l 0.03 0.24 0.019 0.006 0.0044 0.006 0.038 0.10 L 0.006 0.01 0.230.015 0.011 0.0051 0.005 0.043 0.04 M 0.008 0.18 0.30 0.12 0.008 0.00470.005 0.091 0.04 N 0.010 0.35 0.74 0.010 0.010 0.0036 0.008 0.034 0.16

Table 3a Nb 2 Ti Nh Zr 7 r T 6 W Mo Cr Cu Control ,c 0oo1 steel Steel DI 0.028 of, this E 0.10 0.013 invention F 0.12 0.016

' I O 0.05 0.08 0.032 H 0.02 0.04 0.070 l 0.01 0.02 0.034 0.05

L 0.02 0.08 0.023 0.23 0.17 M 0.02 0.10 0.026 N 0.04 0.04 0.050 0.080.07 (H4 0.26

Table 4 Conical cup value in Example Condition of manufacture Conicalcup value (mm) Fishing Heat treat- Control temperament after steel Steelof this invention ture rolling C D E F G H l .1 K L M N Control 900CNone (Coiled 86.3 85.6 85.5 85.7 85.5 86.1 85.4 85.6 85.8 85.7 85.8 86.2method at 650C) Table 4 Continued Conical cup value in Example Conditionof manufacture Conical cup value (mm) Fishing Heat treat- Control ltemperament after steel Steel of this 1nver1t1on ture rolling C D 1. F(l H l .l K L M N Method 650C Box annealing 87.6 77.2 77.0 77.1 76.978.3 76.3 75.9 76.8 77.0 76.1 77.9 of this 550C 700C X 4 hr. 86.4 75.074.3 74.8 74.2 76.4 74.1 73.9 74.0 75.2 74.2 75.6 inven- 740C Continuous89.1 85.4 85.2 85.3 85.1 85.4 84.7 84.7 85.1 85.0 85.1 86.4 tion 650Cannealing 88.1 76.8 77.7 77.7 76.7 77.3 76.2 75.7 75.9 76.8 75.9 77.2550C 850C X 5 min. 86.3 74.0 74.7 75.5 74.0 75.6 73.9 73.4 74.0 75.173.7 75.8

Thickness: 2.7 mm (onicnl cup blank diz1.; 108 mm Table 5 Strength ofX-ray reflection in Example Strength of X-ray reflection {l l 1} axialdensity 100} axial density] Finishing Control tempera- Heat treatmentsteel Steel of this Invention ture after rolling C D. 1 E, J F, K G, L

Control 900C None (Coiled at 1.0/ 1.1 1.1 1.2 1.0/ 1.0 1.0/ 1.1 1.2 1.]method 650C) '1 740C 1.5/3.0 2.0/1.3 2.1/1.3 Method 650C Box annealing2.0 1.9 7.0/ 1.0 6.9 0.9 of this 550C 700C X 4 hr. 2.6/ 1.6 7.5 1.0 7.8/1.0 inven- 740C Continuous annealing 1.7 3.3 2.1 1.2 2.3 1.3

tion 650C 850C X 5 min. 2.1 1.8 7.4/ 1.1 7.2/ 1.0 550C 2.5/1.4 7.4/l.07.6/1.1

'1 not measured What is claimed is:

1. A method for manufacturing a steel sheet having an excellent pressformability which comprises heating 21 killed steel composed of not morethan 0.10% C, 0.05% to 1.0% Mn, 0.005% to 0.15% A1, at least one memberof the group consisting of Ti, Nb and Zr, and the rest Fe andunavoidable impurities, the Ti%/4 -l- Nb%/7.8 Zr%/7.6 being more thanthe C in the steel, subjecting the same to hot rolling such that thefinishing temperature is 750C or less, and thereafter effecting atreatment for recrystallization.

2. A method according to claim 1 in which said killed steel furthercontains at least one member selected from the group consisting of notmore than 0.15% P, not more than 0.10% W, not more than 0.10% Mo, notmore than 0.3% Cr, not more than 0.3% Cu and not more than 0.4% Si.

3. A method according to claim 1 in which the total reduction rate below800C is at least 40% of the thickness of the material at 800C.

4. A method according to claim 1 in which C is no greater than 0.03%; Mnis 0.1% to 0.5%; and Al is 0.01% to 0.15%.

5. A method according to claim 1 in which the finishing temperature is400C to 750C.

6. A method according to claim 1 in which the recrystallizationtreatment is conducted by heating at temperatures ranging from therecrystallization temperature up to A point.

7. A method according to claim 1 in which the recrystallizationtreatment is conducted by box annealing, open coil annealing orcontinuous annealing.

8. A method according to claim 2 in which the total reduction rate below800C is at least 40% of the thickness of the material at 800C.

9. A method according to claim 2 in which C is no greater than 0.03%; Mnis 0.1% to 0.5%, and Al is 0.01% to 0.15%.

10. A method according to claim 2 in which the finishing temperature is400C to 750C.

11. A method according to claim 2 in which the recrystallizationtreatment is conducted by heating at temperatures ranging from therecrystallization temperature up to A point.

12. A method according to claim 2 in which the recrystallizationtreatment is conducted by box annealing. open coil annealing orcontinuous annealing.

13. A method according to claim 1 in which the ratio of 111 axialdensity axial density is within the range from about 2.0/1.3 to about7.8/1.0.

14. A method according to claim 2 in which the ratio of l l l axialdensity {100} axial density is within the range from about 2.0/1.3 toabout 7.8/1.0.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,897,280 Dated July 29, 1975 Inventor(s) Hisashi Gondo, et al It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Col. 5, lines 23 and 24 should read perature, not only is the strain notfully released but also an aggregative structure desirable for deep-.

Col. 6, line 18, after "having" insert --been- Table 5, the second linethereof should read Strength of X-ray reflection lll} axial density/Col. 8, line 52, "111" should read {111} Col. 8, line 55,- "111" shouldread {111} Signed and Sealed this sixteenth D a y 0 f March 19 76 [SEAL]A ttes t:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner ofPatemsand Trademarks

1. A METHOD FOR MANUFACTURING A STEEL SHEET HAVING AN EXCELLENT PRESSFORMABILITY WHICH COMPRISES HEATING A KILLED STEEL COMPOSED OF NOT MORETHAN 0.10% C, 0.05% TO 1.0% MN, 0.005% TO 0.15% AL, AT LEAST ONE MEMBEROF THE GRUP CONSISTING OF TI, NB AND ZR, AND THE REST FE AND UNAVOIDABLEIMPURITIES, THE TI%/4 + NB%/7.8 + ZR%/7.6 BEING MORE THAN THE C % IN THESTEEL, SUBJECTING THE SAME TO HOT ROLLING SUCH THAT THE FINISHINGTEMPERATURE IS 750*C OR LESS, AND THEREAFTER EFFECTING A TREATMENT FORRECRYSTALLIZATION.
 2. A method according to claim 1 in which said killedsteel further contains at least one member selected from the groupconsisting of not more than 0.15% P, not more than 0.10% W, not morethan 0.10% Mo, not more than 0.3% Cr, not more than 0.3% Cu and not morethan 0.4% Si.
 3. A method according to claim 1 in which the totalreduction rate below 800*C is at least 40% of the thickness of thematerial at 800*C.
 4. A method according to claim 1 in which C is nogreater than 0.03%; Mn is 0.1% to 0.5%; and Al is 0.01% to 0.15%.
 5. Amethod according to claim 1 in which the finishing temperature is 400*Cto 750*C.
 6. A method according to claim 1 in which therecrystallization treatment is conducted by heating at temperaturesranging from the recrystallization temperature up to A3 point.
 7. Amethod according to claim 1 in which the recrystallization treatment isconducted by box annealing, open coil annealing or continuous annealing.8. A method according to claim 2 in which the total reduction rate below800*C is at least 40% of the thickness of the material at 800*C.
 9. Amethod according to claim 2 in which C is no greater than 0.03%; Mn is0.1% to 0.5%, and Al is 0.01% to 0.15%.
 10. A method according to claim2 in which the finishing temperature is 400*C to 750*C.
 11. A methodaccording to claim 2 in which the recrystallization treatment isconducted by heating at temperatures ranging from the recrystallizationtemperature up to A3 point.
 12. A method according to claim 2 in whichthe recrystallization treatment is conducted by box annealing, open coilannealing or continuous annealing.
 13. A method according to claim 1 inwhich the ratio of (111) axial density (100) axial density is within therange from about 2.0/1.3 to about 7.8/1.0.
 14. A method according toclaim 2 in which the ratio of (111) axial density (100) axial density iswithin the range from about 2.0/1.3 to about 7.8/1.0.